There have been many designs proposed for the transfer of cargo between the cargo swivel of a single point mooring system and the moored tanker. Most single point mooring systems now in use employ floating cargo hose which connects to a cargo swivel mounted on the deck of a floating mooring buoy anchored by catenary chains as typically disclosed in U.S. Pat. No. 3,074,082. In that system the floating cargo hose experiences excessive wear due to flexing in the wave environment, especially at the point of connection between the hose and the buoy, and the floating cargo hose is exposed to damage by the moored tanker especially if the tanker should move forward and strike the buoy. Furthermore, the floating hose is expensive and its size limits cargo transfer rates.
Some recent single point mooring installations have used the more advanced single anchor leg mooring design disclosed in U.S. Pat. Nos. 3,606,397 and 3,641,602, assigned to the assignee of the present invention, in which the mooring buoy is anchored by a single anchor leg and the hose connects to a submerged cargo swivel which is concentric with the anchor leg and mounted either on a mooring base or on a shaft pivoted on the mooring base. In this design the problem of excessive hose wear at the buoy connected and the danger of cargo system damage resulting from a tanker impacting the buoy are eliminated.
There have been several designs proposed which are like the single anchor leg mooring and incorporate underwater rigid pipe loading arms such as that disclosed in U.S. Pat. No. 3,455,270, also assigned to the assignee of the present invention. However, it has been determined that in such cargo transfer systems the distance between the mooring point and the end of the rigid pipe loading arm is fixed, and the moored tanker cannot be permitted to move too far forward or aft of the mooring without exerting excessive forces on the loading arm.
More recently a single anchor leg mooring having an underwater articulated pipe loading arm including three pivoted segments and a loading tower pivoted at the end has been proposed as disclosed in the paper "Offshore Floating Terminals," Proceedings of the American Society of Civil Engineers WW3, August 1971. This design has the advantage that the distance between the mooring point and the loading tower at the end of the loading arm is not fixed, and the moored tanker may move forward and aft without exerting excessive forces on the loading arm, as demonstrated by model tests conducted on the system. However, these model tests have also demonstrated that the loading tower pivoted at the end of the loading arm moves around violently in waves, is difficult to constrain along the side of the tanker, and may strike the tanker resulting in damage to the tanker and the loading tower. One possible design for such a system would require extensive modifications to the tanker side and rail in order to provide strong points against which the loading tower would rest and to which the tower would be attached. Such modifications would be impractical to make on all tankers, and any such cargo handling system then would be limited to serving only a few specially modified tankers.
Furthermore, this earlier articulated loading arm is considered unstable because its three arm segments can move uncontrolled to any position between the end pivot points at the base and the tanker side. It is even possible for a pivot between two adjacent arms to pass under the keel of the moored tanker which could result in damage to the tanker or the arm.